1. Field of the Invention
The invention relates to cathode ray tube display devices suitable for use in high ambient light level conditions and, in particular, concerns an optical filter arrangement for enhancing the brightness of the red and green spectral line display generated by a penetration phosphor cathode ray display device.
2. Description of the Prior Art
Cathode ray display tubes satisfactory for application in information displays such as air traffic control, radar, data processing, and the like systems have unusual requirements generally not met by cathode ray tubes of the type conventionally available for color television viewing. In particular, they often do not require the many colors needed for a television screen; for example, the provision of blue may even be undesirable. While red and green are readily simultaneously focused by the optics of the normal eye at the sensitive high resolution fovea, the blue component of the image is normally out of focus and its presence may even tire the eye. Further, the very center of the fovea is red and green sensitive, but blue is not appreciably sensed there. Since red and green light may purposely be mixed so as to yield the sensations of orange or of yellow in the eye, a wide range of colors adequate for information displays is available without the use of blue.
Further features are desirable in an information display; a major need is to provide brightness and contrast when the display is viewed in a high ambient light level, including sunlight, whereas conventional color cathode ray television tubes are useful only at low or medium light levels. High resolution, especially in the center of the screen, not available in conventional tubes, is also desired. Relatively simple electron beam focusing and scanning elements are also desired so that either raster scanning or random beam positioning as both often required for presenting computer information are easily achieved.
The penetration phosphor cathode ray display tubes of the prior art offer several of the advantages required in specialized information displays, but the most suitable penetration phosphor indicator for use in information displays is that disclosed by S. F. Ignasiak in patent application Ser. No. 505,710, filed Sept. 13, 1974 for "Penetration Phosphors and Display Devices" and assigned to Sperry Rand Corporation. The penetration phosphor color display of the Ignasiak invention overcomes many defects inherent in prior art color displays, again permitting much more information to be displayed effectively simultaneously than by a black and white tube. Additionally, recognition of individual data representations in the displayed material when colored is much more rapid than in a monochromatic or black and white display.
Full use may be made of the penetration phosphor characteristics of the Ignasiak invention in providing a variable color display while using only one electron beam simply by controlling the voltage on the beam acceleration electrode adjacent the cathode. With red and green emitting phosphors, successive changes in the beam acceleration voltage generate distinct colors; for example, red, orange, yellow, or green may thus be generated. Only one suitably controlled electron gun is required and the complexities of prior art color television tubes are eliminated along with other features of prior art display devices undesirable in information displays.